If we only want the central petals of this rose to be in focus we can easily arrange for this to happen in a few simple steps using a layer mask and Gaussian blur to recreat a similar affect that depth-of-field has on focusing. Following this Gimp tutorial you will learn how to select the subject and then make the background appear out of focus.
Step 1 As always our first act is to duplicate the background layer and rename it, here we want three layers; the original background image, the layer we are going to blur and a third layer that will consist of the areas we want to remain in focus.
Place the ‘blur’ layer at the top of the list, we could do this with the blur underneath the focussed layer, but this would mean more work when it comes to the layer masking stage.
Step 2 Now that we have our layers prepared, enter the ‘blur’ layer and open the filters menu.
Now, move through the ‘blur’ option to select Gaussian Blur. For this image a blur of around 20 pixels should suffice, but we can always apply more on top of the result if we want an even stronger effect.
Step 3 Now apply a layer mask to the blur layer by right-clicking it in the Layers Pane and choosing ‘Add Layer Mask’. When promoted select ‘White (Full Opacity)’, which will leave the layer fully visible.
Step 4 Select a soft-edged Paintbrush in the left pane, brush 19 is a personal favourite for this sort of job, and make sure that black is your foreground colour by pressing ‘D’ which restores your selected colours to the default choices. You can switch between black and white as your foreground colour on the fly by pressing ‘X’ at any time, which is useful when applying a layer mask as it allows you to quickly correct any mistakes you may make.
Ensure that you have the layer mask selected in the Layers pane before you start, then paint with the black paintbrush the areas you want to have in focus. Use white to re-blur areas you have cleared, and remember that you can use the left and right square bracket keys to change the size of your brush as you work.
Step 5 To really make the effect believable we need to make the transition between the degrees of focus less instantaneous; rather than having areas either in or out of focus we need some parts of the image that are somewhere in between. To achieve this effect we edit the layer mask, but instead of using black or white use a mid grey to paint areas that are between those that you want in focus and the areas that would be completely out of focus.
This will merge the two levels of focus and make the image more acceptable to the eye. There you have it, a fully intuitive way of applying a depth of field effect to any part of a photograph you want, you can further increase the control you have over the effect by using more shades of grey to control the layer mask, with darker values making the blurred layer less apparent and lighter shades allowing to show more.